In recent years, with the use of nanometer-size particles, there have been proposed ultramicro electronic devices such as single-electron transistors, single-electron memories, and memories in which fine particles called nanodots or nanocrystals are contained in the gate insulator. Memories of this kind are expected to operate with low power consumption by taking advantage of a quantum size effect of Coulomb blockade phenomenon or the like.
However, conventional single-electron transistors or single-electron memories are difficult to integrate because very fine processing is required to fabricate a nanosize dot capable of storing just one or several electrons and to detect a flow of several electrons. Moreover, in many cases, there has been a need for keeping the memories or the like at extremely low temperature in order to suppress the occurrence of malfunctions due to thermal fluctuations. Therefore, memory devices utilizing the Coulomb blockade phenomenon or the like lack practicability and stay at an experimental level.
As shown in FIG. 62, a conventional memory device that employs particles in its floating gate has, on a channel region located between source and drain regions 2806 formed in a p-type silicon substrate 2801, its components including an oxide film 2802 that has a thickness of 2 nm and is formed by thermal oxidation, silicon particles 2803 that have a particle size of 5 nm and are formed on the oxide film, an oxide film 2804 formed so as to cover the silicon particles and a polysilicon layer 2805 that serves as a gate electrode.
As a method for forming the silicon particles 2803 in an insulation film, there is proposed a method for depositing amorphous silicon on the silicon thermal oxidation film 2802 by an LPCVD (Low-Pressure Chemical Vapor Deposition) apparatus, thereafter forming the silicon particles 2803 through an annealing process and further depositing the silicon oxide film 2804 on the silicon particles 2803 by a CVD (Chemical Vapor Deposition) process (see, e.g., JP 2000-22005 A).
As a method for forming particles as shown above, there are proposed a method for forming crystals on a substrate by using CVD, evaporation, MBE (Molecular Beam Epitaxy) or the like and a method for forming a thin film and thereafter using etching or other fine processing technique. In such methods, after particles are formed, an insulator layer is stacked on the particles.
With memory devices of this kind, it is often the case that the surface density of particles is insufficient or that the scale-down of the particle size is insufficient. As a result, there are disadvantages such as narrow memory windows, larger variations and poor retention characteristics.
With the method of forming the particles by using the CVD, evaporation, MBE and so on, the particles can be formed only on one surface through one-time process in order to raise the surface density. Therefore, it has been the case that similar process needs to be repeated many times.
Also, with the method of using the fine processing technique of photolithography, etching and the like, it is extremely difficult to reduce both particle size and particle-to-particle distance to the nanometer order at the same time.
Meanwhile, there have not yet been known any resistor devices in which a multiplicity of conductive particles, which are fabricated at once, are nanometer-size particles (nanodots) superior in thermal stability.